Thick film materials generally are mixtures of metal, glass and/or ceramic powders dispersed in an organic vehicle. These materials are applied to substrates to form conductive, resistive or insulating films. Thick film materials are used in a wide variety of electronic and light electrical components.
The properties of individual compositions depend on the specific constituents which comprise the compositions. All compositions contain three major components. The conductive phase determines the electrical properties and influences the mechanical properties of the final film. In conductor compositions, the conductive phase is generally a precious metal or mixture of precious metals. In resistor compositions the conductive phase is generally a metallic oxide. In dielectric compositions, the functional phase is generally a glass or ceramic.
The binder is usually a glass, a crystalline oxide or a combination of the two. The binder holds the film together and to the substrate. The binder also influences the mechanical properties of the final film.
The vehicle is a solution of polymers in organic solvents. The vehicle determines the application characteristics of the composition.
In the composition, the functional phase and binder are generally in powder form and have been thoroughly dispersed in the vehicle.
Thick film materials are always applied to a substrate. Thick film technology is defined as much by the processes as by the materials or applications. The basic thick film process steps are screen printing, drying and firing. The thick film composition is generally applied to the substrate by screen printing. Dipping, banding, brushing or spraying are occasionally used with irregular shaped substrates.
The screen printing process consists of forcing the thick film composition through a stencil screen onto the substrate with a squeegee. The open pattern in the stencil screen defines the pattern which will be printed onto the substrate.
After printing, the film is dried and fired, generally in air. This process forms a hard, adherent film with the desired electrical and mechanical properties.
Additional thick film compositions may be applied to the same substrate by repeating the screen printing, drying and firing processes. In this way, complex, inter-connected conductive, resistive and insulating films can be generated.
One particular application of thick film materials are thick film heater compositions which are widely used in the electronics industry. Heaters can be fired onto a variety of substrates such as 96% alumina, aluminum nitride, stabilized zirconia and 430 grade stainless steel.
High temperature heaters, for instance, for oxygen sensors that are required to operate above 600° C. to 700° C., typically are based on platinum metal as the conductor element. Alternately, high temperature heaters may employ a base metal such as tungsten or molybdenum co-fired within an alumina package.
U.S. Patent Application Publication 2010/0101950A1, entitled “Ceramic Junction Member, Ceramic Heater and Gas Sensor” assigned to NGK Spark Plug Co, Ltd., discloses W and Mo heaters used with alumina. U.S. Pat. No. 7,280,028, entitled “Temperature Sensor and Method of Making the Same, assigned to Delphi Technologies, inc., discloses disposing a film of platinum, rhodium, palladium and mixtures and alloys thereof on a substrate to provide a fast action O2 sensor. U.S. Pat. No. 5,898,360 entitled “Heater for Heating an Automobile Sensor, assigned to Samsung Electro Mechanics, Inc., discloses a ceramic heater with improved durability having an electrode made from platinum and at least one lanthanide oxide.
U.S. Pat. No. 6,444,297, entitled “Method of Producing a Thick Film Metallization on an Aluminum Nitride Substrate”, assigned to Electro Technik Industries, Inc., discloses a metallization layer structure is applied to an aluminum nitride substrate by the application of an intermediate buffer layer of either silicon monoxide or silicon dioxide. Additionally, a resistive thick film, such as described in U.S. Pat. No. 4,539,223, may be added to the metallization layer.
Lower temperature heaters, on the other hand, can be based on silver as the metallization. One example is a kettle heater (see “DuPont Users' Guide for DuPont HEATEL® Inks For Heating Applications on Steel Substrates”).
Heating elements used in ceramic heaters, cooking appliances and the like are also disclosed in U.S. Patent Application Publications 2004/0094533, 2005/0016986 and 2009/0134144, U.S. Pat. Nos. 4,845,340 and 6,046,438, and European Patent Specification EP 0958712. None of the systems disclosed use a combination of Ag and RuO2.
Silver-based heaters have the dual benefits of lower cost vs. platinum heaters, and the ability to be air fired compared with tungsten or molybdenum heaters that require controlled atmosphere firing. Such heaters can be fabricated with resistor of various conductivities, including a resistivity of approximately the bulk value of the conductor employed. However, there are needs in the industry for sheet resistances higher than that of the pure metal, e.g., silver. Accordingly, one difficulty with the approach of using silver as the conducting material is that it has a sheet resistivity of around 1.6 milliohms/square when fired to 10 microns thickness. Furthermore, with thick film heaters based on low-cost silver compositions, there can be difficulties in controlling the resistance of the fired layers.
In considering the need for a higher ohm, silver-based commercial heater resistor compositions, the inventors have considered this may require substantial modification of the silver-containing composition to raise the sheet resistance. Although it was considered that such modification could be done by dilution of silver with glass and possibly other additives, there are considerable difficulties and problems encountered in controlling the resistivity of the silver-based heaters. As the sheet resistivity is formulated to higher values, the coefficient of variation (CV) typically increases. Also, quite unexpectedly, the fired heater resistors show a sensitivity to print thickness that is more severe than an inverse thickness calculation would predict. Although not wishing to be bound by any particular theory or hypothesis, this thickness sensitivity and undesirable variability of the resistivity value is thought to possibly be due to silver sintering and coarsening as the printed film increases in thickness.
This problem, as described above, is not discussed in U.S. Pat. No. 5,304,784 which discloses that a heater may be formed of silver-palladium paste, or alternately, of RuO2 paste; U.S. Pat. No. 5,162,635 which discloses an electrically conductive section 24 made of RuO2 or silver; U.S. Pat. No. 5,083,168 which discloses that resistor 22 may be made from silver/palladium, RuO2 or nickel; or U.S. Pat. No. 5,068,517 which discloses that a strip heater element 22 may be made from only silver-palladium alloy, or a mixture of the silver-palladium alloy and RuO2. The strip heater element 23 is covered with a protective layer 24 formed by coating with frit glass and backing the frit glass.
The problem solved by the present invention is not discussed in U.S. Pat. No. 6,406,646, which discusses the disadvantages of the processes and paste compositions disclosed in JP 53-100496, U.S. Pat. No. 5,510,823 and Korean Patent No. 130831 issued to DuPont because they require a high calcination temperature ranging from about 600 to 1,000° C. thus limiting their application to substrates which can stand such a high temperature. U.S. Pat. No. 6,406,646 discloses a resistive paste composition comprising Ru metal or RuO2 particles, silver(Ag) metal or compounds, a glass frit having a softening point of 400 to 550° C. and an organic binder. The paste is calcined at a low temperature of about 500 to 600° C.
U.S. Pat. No. 5,510,823 discusses JP 53-100496 in great detail and concludes that process has numerous disadvantages, including that the thick resistive element film-forming paste is an non uniform mixture of a glass frit powder and a ruthenium oxide powder, the resulting resistive value varies widely or the strength to electric field is low, the resistive value suddenly changes with a change in applied voltage, it is difficult to control the resistive value of the resulting resistive element by the composition ratio of a glass powder and a ruthenium oxide powder alone, and the difference in grain diameter between glass powder and ruthenium oxide powder or the variation of calcining temperature causes a great dispersion of resistive value. It is further stated that even if the composition ratio and the average grain diameter are kept constant, the resistive value of the resulting resistive elements of JP 53-100496 differ greatly. U.S. Pat. No. 5,510,823 relates to a resistive element for use in various electronic components such as hybrid integrated circuit and thermal head and proposes and claims a resistive element film-forming paste, which comprises (1) an organic metal compound, (2) at least one additive selected from organic nonmetal compounds and organic metal compounds, and (3) a solution of asphalt dissolved in a solvent.
Commonly assigned U.S. Pat. No. 5,162,062 discloses an invention directed to a technique for developing improved conductor aged adhesion over dielectric layers. It is suitable for both glassy and filled glass dielectric systems, as well as for crystallizing and filled crystallizing systems. It is especially useful for the crystallizing and filled crystallizing type of systems on account of the difficulty in bonding conductors to crystallized glass layers. Disclosed as a part of the method is a patterned layer of thick film conductor paste comprising (a) a silver-containing metal selected from Ag, alloys and mixtures of Ag with a minor amount of Pd and/or Pt, (b) an amorphous glass binder, (c) a sintering inhibitor selected from oxides of ruthenium and rhodium, mixtures and precursors thereof, all of (a), (b), and (c) being dispersed in an organic medium.
Despite all of the considerable efforts described above, a need exists for compositions and methods for forming silver based heaters having sheet resistances higher than that of the pure silver metal, that is, which are able to be consistently and uniformly processed to raise the sheet resistivity to higher values, with acceptable control of HTCR values, and without negatively impacting the coefficient of variation (CV) and furthermore without unpredictable sensitivity to print thickness. Moreover, a need exists for methods and compositions that can produce resistors that are more uniform in fired performance, and can yield a resistivity that varies more closely with the inverse print thickness in order to more easily predict the fired heater resistance based upon the actual print thickness employed.